Gas Infrared Ovens.

 
 

Infrared ovens may be either electric infrared or gas infrared. We have the unique ability to discuss both: Because we use both. We have a wealth of infrared experience to assist you.

Gas infrared ovens use gas burners, or sources. Gas infrared sources typically burn a fuel/air mixture to heat a metal or ceramic to an incandescent temperature. The following descriptions highlight the most common sources for gas infrared ovens.

High intensity impingement burners. Impingement burners, available in a variety of shapes and sizes, radiate flame energy from refractory ceramics of various shapes. Typically, the sources offer a source temperature of 1,200 to 1,900°F and a heat intensity of 10,000 to 50,000 BTUs per hour per square foot. They typically appear red to orange during operation, heat up in 15 to 90 seconds, are resistant to thermal and physical shock, and have a typical radiant efficiency of 30 to 50%.

Impingement burners are generally not susceptible to preignition, or flashback, and resist physical damage but have relatively low radiant efficiencies. They effectively resist damage from falling parts and are also effective in multipass ovens where cooling air cannot be provided at the rear of the burner. Life expectancies are measured in years, unless the burner is subjected to overfiring.

High intensity porous matrix burners. Porous matrix burners have porous or perforated refractory plates mounted on cast iron or formed steel plenum chambers. The refractory material may be porous ceramic, refractory blanket, ported ceramic, stainless steel, or metallic screens. Typically, like impingement burners, the porous matrix sources offers a temperature of 1,200 to 1,900°F and an intensity of 10,000 to 50,000 BTUs per hour per square foot. They typically appear red to orange during operation, heat up in 15 to 90 seconds. Recently developed porous matrices of woven ceramic fiber can heat-up and cool-down in seconds. Porous matrix burners are resistant to thermal and physical shock, and have a typical radiant efficiency of 30 to 60%. Porous matrix burners typically have the highest radiant efficiency of the gas-fired infrared sources.

The fuel and air mixture, supplied under pressure to the plenum chamber, passes through the porous matrix to burn on the surface facing the load. Combustion occurs evenly on the exposed surface, heating it to incandescence. As the surface heats up, the flame recedes into the matrix, which adds radiant energy to the flame. Modulating the fuel input provides about a 3:1 turndown capability in oven heating intensity. (Turndown is a ratio of the maximum and the minimum temperature.) Airflow, as a result of natural convection, usually cools the plenum chamber on the rear of the burners to prevent pre-ignition of the combustible mixture.

Catalytic sources. Catalytic sources consist of a porous ceramic material, or blanket, impregnated with a catalyst, such as platinum black, through which a combustible air/gas mixture, or gas alone, is fed. Catalytic burners are similar to porous matrix burners in construction, appearance, and operation, but the refractory material is usually glass or ceramic wool. Typically, the sources offer a temperature of 600 to 1,000°F and an intensity of 2,000 to 7,500 BTUs per hour per square foot. They typically appear to have little color during operation, heat up in 180 to 600 seconds, are resistant to thermal and physical shock, and have a typical radiant efficiency of 30 to 75%. The combustible air and gas mixture oxidizes within the catalytic matrix at temperatures below those normally required for combustion. As a result, zoning is simplified because fewer safety devices are required. No visible flame is produced. As a result, these burners provide low-to-moderate intensities. And the burners must include an alternate heat source, usually electric heating elements, to preheat the catalyst before operation with gas.

Radiant tube and panel sources. Radiant tube or panel sources are internally fired metal tubes or panels. Radiant tubes have a burner at one end firing down the tube. Typically, the sources offers a temperature of 400 to 1,200°F and an intensity of 1,000 to 10,000 BTUs per hour per square foot. They typically operate at surface temperatures up to 1,200°F. They typically appear red or have little color during operation, heat up in 300 to 600 seconds, are resistant to thermal and physical shock, and have a typical radiant efficiency of 25 to 50%. Radiant panel ovens, sometimes called "radiant walls", surround the parts to be heated with a metal enclosure. Radiation and hot combustion by-products, scrubbing the surface of the enclosure, heat the exterior of the enclosure facing the load. Infrared emission heats the parts in the interior of the enclosure. Combustion by-products can be vented or ducted to the convection portion of the oven.

Gas infrared ovens are common in a variety of applications. Impingement and porous matrix burners are great for ovens requiring large amounts of heat input, like powder coating of large, heavy parts. Catalytic burners are great for liquid coating applications and thermoforming. Regardless of the application, all well designed gas infrared ovens exhibit the following characteristics:

• Precise layout and distribution of sources. To incorporate shape factors, overcome any edge effect, and provide greater flexibility and zoning.

• Insulated reflective panels to reradiate heat. To provide reradiation even when panels may be dirty.

• Non-contact temperature sensors for control. To provide the optimum in temperature control.

• Custom control. To meet the specific needs of the process and the operators.

• Safety. Adherence to all FM, IRI, OSHA and particularly all NFPA standards and regulations.

See also electric infrared ovens.